Yes, you can make a payment and reservation of a guest spot without having a Harba ID sticker attached to your boat. Be sure to get one at the first marina you use. Then, put it on the front of your boat and make sure it's visible. This way the marina staff will be able to note your boat's ID inside the app. However, if your boat is docked with the stern facing the bridge, the docking staff also have the name of your boat and will often be able to identify your payment through the app.